Metals such as transition metals, including noble and base metals are used as catalysts in many chemical reactions. Catalysts generally increase the rate of chemical reaction which results in higher production rate in industry. Some catalysts are also used to drive reactions along a desired path, i.e., the catalysts make formation of certain chemicals energetically favorable over other chemicals.
In general, catalysts are expensive, and hence it is necessary to utilize a given amount of catalyst to its maximum potential. This is done by maximizing the catalyst surface area, i.e., by increasing its dispersion. Catalysts can be used as solids or liquids. The solid catalysts, although they can be used as powders, are generally supported on high surface area supports. The properties of the support become very important in such cases.
Activated carbon has also been used as a support for catalyst metals, e.g., noble metals because of its very high surface area. Such catalysts are used (powder or beads form) in various petrochemical reactions. These catalysts are normally made by dispersing noble metal particulates on preformed activated carbon (incipient wetness technique).
The incipient wetness technique involves dispersing the activated carbon powders in a solution of a metal salt. The activated carbon powder is then impregnated with the solution. The powder is filtered out, dried, and heated to appropriate temperature to decompose the salt to the desired metal or metal oxide catalyst. Multiple impregnations are usually required to obtain the desired quantity of catalyst on the activated carbon. Surface properties of activated carbon powders play a very important part in the dispersion of the metal catalyst obtained. Oxygen content and surface pH of the carbon powder has to be carefully controlled to obtain a good dispersion of the metal on the activated carbon. The various steps which are involved in this process result in a very expensive activated carbon supported catalyst powder.
Another technique for making activated carbon supported catalysts involves depositing a catalyst metal precursor with high vapor pressure onto a carbon surface. In this technique the carbon surface chemistry affects the choice of catalyst precursors. Often the appropriate precursors are not available for many of the most desirable catalysts. Examples of catalyst metals that cannot be readily vapor deposited are alkaline earths, Cr, Mn, Cu, etc. since compounds of these metals have low vapor pressures. Other metals such as Pt, Pd, and Co are difficult because the volatile compounds of these metals decompose rapidly in the vapor phase. Furthermore, many of those that are available are highly toxic. Finally the method requires highly developed, expensive deposition equipment.
Hence, it would be an advancement in the art to have a simpler and more cost effective method of making an activated carbon catalyst having a catalyst uniformly dispersed thereon.
The present invention provides such a method.